1. Field of the Invention
This invention relates to a steering system for a vehicle, and more particularly to a four-wheel steering system for a vehicle in which the rear wheels are arranged to be turned in response to the turning of the front wheels when the steering wheel is turned.
2. Description of the Prior Art
Conventionally, in four-wheeled cars, generally only the front wheels are turned in response to operation of the steering wheel. However, there has been proposed a four-wheel steering system in which the rear wheels are turned together with the front wheels in response to operation of the steering wheel in order to improve steering performance and running stability and to facilitate the steering of the vehicle.
For example, when the rear wheels are turned in the same direction as the front wheels during travel at a relatively high speed, the vehicle body can be directed substantially tangential to the turning circle without a delay in phase from the turning operation of the steering wheel because side force acts simultaneously on both the rear and front wheels, whereby lanes can be changed smoothly during high speed travel. Further, by turning the rear wheels in the reverse direction to the front wheels when the vehicle is traveling at very low speed, the orientation of the vehicle body can be changed considerably, and accordingly the vehicle can be easily driven into a small space or a garage.
Taking into account the fact that generally it is not during high speed travel but during relatively low speed travel that the steering wheel or the front wheels are turned by a large amount, the four-wheel steering system may be arranged to turn the rear wheels in the same direction as the front wheels when the turning angle of the front wheels is small and in the reverse direction when the turning angle of the front wheels is large.
Theoretically, such a four-wheel steering system can be realized in various ways. For example, there have been proposed various four-wheel steering systems in which the rear wheels are turned in response to operation of the steering wheel by way of electrical means or hydraulic means. However, the four-wheel steering systems in accordance with the conventional proposals are generally disadvantageous in that the structure is apt to be complicated and costly, and provisions for ensuring reliabilities in safety upon occurrence of failure, maintenance and the like are required. In order to simplify the structure and improve reliability, there have been proposed various four-wheel steering systems in which the movement of the steering system for the front wheels is transmitted to the steering system for the rear wheels by way of mechanical means as a force for controlling the movement of the rear wheels. For example, there is disclosed in U.S. Pat. No. 4,295,657 a four-wheel steering system in which a front wheel steering system and a rear wheel steering system are mechanically connected by way of a link and a cam mechanism so that the rear wheels are turned in response to the turning of the front wheels and the steering ratio of the rear wheels is changed with the steering angle of the front wheels. However, in link mechanisms or cam mechanisms, slight play or clearance must generally be provided at each junction between components in order to ensure smooth movement of the components and accordingly play in the turn of the rear wheels with respect to the turn of the front wheels becomes large, giving rise to fluctuation in the turning direction of the rear wheels, delay in turning of the same and various other drawbacks.
Further, in the four-wheel steering systems disclosed in U.S. Pat. No. 4,467,885, a rack-and-pinion type steering device for the front wheels is provided with an additional pinion in mesh with the rack and rotation of the additional pinion is transmitted to a rear wheel steering system by way of a transmission shaft. This arrangement is advantageous over the aforesaid arrangement using a link and a cam mechanism so long as the play in the turn of the rear wheels with respect to the turn of the front wheels can be reduced but involves a problem with the mounting of the additional pinion. That is, because two pinions must be in mesh with a single rack and the position of the original pinion connected to the steering wheel (the position of which cannot be changed) can be hardly changed, the mounting position of the additional pinion is inherently limited. If the steering system for the front wheels is a power-steering system and a power cylinder must be provided on the rack, the mounting position of the additional pinion is further limited. Therefore, there arises a problem with mounting of the transmission shaft for transmitting rotation of the additional pinion to the rear wheel steering system. That is, it is preferred that the transmission shaft is disposed as high as possible so as not to project downward and reduce the minimum ground clearance. Therefore, it is most preferable that the transmission shaft is disposed in the tunnel portion of the vehicle body. However due to the aforesaid limitation in the mounting position of the additional pinion, it is not always possible to dispose the transmission shaft in the tunnel portion. When it is not possible to dispose the transmission shaft in the tunnel portion, a space for accommodating the transmission shaft must be prepared which results in change of the vehicle body shape.